fix(deps) update webrtc-adapter to v7.7.1

Fix Safari RTCSessionDescription.sdp readonly
Based on https://bugs.chromium.org/p/chromium/issues/detail?id=1186456&

package-lock.json

       }
     },
     "webrtc-adapter": {
-      "version": "7.5.0",
-      "resolved": "https://registry.npmjs.org/webrtc-adapter/-/webrtc-adapter-7.5.0.tgz",
-      "integrity": "sha512-cUqlw310uLLSYvO8FTNCVmGWSMlMt6vuSDkcYL1nW+RUvAILJ3jEIvAUgFQU5EFGnU+mf9/No14BFv3U+hoxBQ==",
+      "version": "7.7.1",
+      "resolved": "https://registry.npmjs.org/webrtc-adapter/-/webrtc-adapter-7.7.1.tgz",
+      "integrity": "sha512-TbrbBmiQBL9n0/5bvDdORc6ZfRY/Z7JnEj+EYOD1ghseZdpJ+nF2yx14k3LgQKc7JZnG7HAcL+zHnY25So9d7A==",
       "requires": {
         "rtcpeerconnection-shim": "^1.2.15",
         "sdp": "^2.12.0"

package.json

     "strophejs-plugin-disco": "0.0.2",
     "strophejs-plugin-stream-management": "github:jitsi/strophejs-plugin-stream-management#001cf02bef2357234e1ac5d163611b4d60bf2b6a",
     "uuid": "8.1.0",
-    "webrtc-adapter": "7.5.0"
+    "webrtc-adapter": "7.7.1"
   },
   "devDependencies": {
     "@babel/core": "7.5.5",